A solar cell is a device that converts solar energy (e.g., sunlight) into electricity. Solar cells have many applications. Individual cells may be used for powering small devices, whereas large arrays of cells (e.g., photovoltaic arrays) may be used to generate a form of renewable energy, which may be particularly useful in situations where electrical power from a power grid is unavailable. Solar cell arrays are also now being developed for grid-based electrical systems.
Solar cells operate by generating electron-hole pairs within a substrate (e.g., semiconductor substrate) in response to the absorption of incident photos into the substrate. When a photon is absorbed, its energy is given to an electron in the crystal lattice of the substrate. Usually, this electron is in the valence band of the crystal lattice and is tightly bound in covalent bonds between neighboring atoms. The energy given to the electron by the photon may be sufficient to excite the electron into the conduction band of the crystal lattice, where it then becomes free to move within the substrate. The covalent bond that the electron was previously part of now has one fewer electron, which is known as a “hole”. The presence of a missing covalent bond allows bonded electrons from neighboring atoms to move into the “hole”, leaving another hole behind, and in this way a hole can move throughout the lattice. This movement of electrons and holes within the substrate can then be utilized to establish a dc voltage across a load connected to the solar cells.
In particular, a built-in electric field generated within a p-n junction can be sufficient to cause movement of electrons and holes within electron-hole pairs to an n-type semiconductor region and p-type semiconductor region, respectively. One example of a solar cell that utilizes a p-n junction and a pair of electrodes on opposite surfaces of a semiconductor substrate is disclosed at U.S. Pat. Nos. 4,726,850 and 4,748,130. Another example of a solar cell is disclosed in U.S. Pat. No. 7,335,555 to Gee et al., entitled “Buried-Contact Solar Cell With Self-Doping Contacts”.